A variety of techniques are known for interconnecting microelectronic components, such as when electrically coupling a semiconductor die to a substrate. These techniques include wire bonding, tape automated bonding (TAB) and flip chip bonding. In wire bonding, a bond pad on a microelectronic component is attached to a contact on a substrate or other microelectronic component. In forming the wire bond, a bonding wire is fed through a capillary which is guided under computer control. A free air ball of molten wire will be formed at the tip of the wire. This free air ball will then be pressed against the bond pad of the microelectronic component or the contact of the substrate, forming a “ball bond.” Most commonly, the ball bond is formed on the bond pad of the microelectronic component rather than on the contact of the substrate. The capillary is then guided upwardly to a predetermined height and the wire is kinked before it is bent and guided to be joined to the substrate. The second end of the bonding wire is typically attached to the contact of the substrate using a “stitch bond.” In a stitch bond, ultrasonic energy is delivered to the wire through the capillary as the capillary presses the wire against the contact of the substrate. A bonding head carrying the capillary is retracted to leave a wire tail at the stitch. Thereafter, a wire clamp carried by the bonding head is closed, tearing the wire as the bonding head retracts further. Thereafter, the same process may be repeated to wire bond two different locations. U.S. Pat. No. 6,110,823 (the entirety of which is incorporated herein by reference) provides a detailed discussion of many aspects of wire bonding operations.
Ball bonding requires that a predetermined length of the wire extend outwardly away from the ball bond before the wire is bent. If the wire is bent too close to the location of the ball bond, the ball bond joint will be stressed and can fail. To avoid such failures, the wire must have a minimum “loop height” (commonly about 6 mils or greater) measured from the location of the ball bond up to the top of the wire where it is bent.
In conventional chip-on-board (COB) structures, a semiconductor chip or other microelectronic component may be attached to a substrate with the active surface of the microelectronic component facing outwardly away from the substrate. The back surface of the microelectronic component is adhesively bonded to the substrate. The bond pads on the microelectronic component may then be wire bonded to contacts arranged on the surface of the substrate to electrically couple the microelectronic component to the substrate. If the bonding wire is ball bonded to the: bond pads of the microelectronic component, the wire will have to extend outwardly away from the active surface of the microelectronic component at least the minimum loop height before it can be angled downwardly and stitch bonded to the contact of the substrate. This can materially increase the overall height or profile of the microelectronic device assembly.
The bonding wire may extend laterally outwardly away from a stitch bond rather than generally perpendicularly outwardly, as is the case in a ball bond. The minimum height of a stitch bond, consequently, can be significantly smaller than a ball bond, being limited primarily by the thickness of the bonding wire. The total height of a microelectronic device assembly can be reduced if the bonding wire is ball bonded to the contact of the substrate then stitch bonded to the bond pad on the microelectronic component because the bond wire can extend vertically upwardly the entire thickness of the microelectronic component before being bent. Since most microelectronic components are significantly thicker than the minimum loop height for a stable ball bond, this avoids any undue stress on the ball bond.
Stitch bonding a wire directly to a bond pad on a microelectronic component can be problematic, though. The stress on a microelectronic component is relatively low during ball bonding operations because the molten ball which is pushed into contact with the bond pad can flow outwardly to create the “nail head” without placing undue stress on the bond pad itself. The bond pads and integrated circuitry of semiconductor chips are most commonly formed of aluminum, though copper is gaining increased acceptance. Most bonding wires are formed of gold. To get a strong metallurgical bond between the gold bond wire and aluminum bond pad, for example, sufficient compression force and ultrasonic energy must be applied with the capillary to form gold-aluminum intermetallics. This places a lot of stress on the aluminum bond pad, which may damage the bond pad or the underlying circuitry.
FIGS. 1 and 2 schematically illustrate one current technique used to minimize the problems associated with stitch bonding to an aluminum bond pad of a semiconductor die. This technique, conventionally referred to as “stand-off stitch bonding” (SSB), the wire 4 is not directly stitch bonded to the bond pad 2 of the die 1. Instead, a ball bump 3 is first applied to the bond pad 2. A short length 5 of the bonding wire 4 is then stitch bonded to the ball bump 3. The ball bump 3 is usually formed as an extra step in the wire bonding process. A free air ball of molten metal will be formed at the end of the wire and the ball will be pressed against the bond pad 2 using the capillary. Rather than continuing to feed the wire as the capillary moves toward the second connection, the bonding wire is torn just at the top of the ball bond, leaving a coined ball bump 3 on the bond pad 2. The wire is then extended, the tip is melted to form a new free air ball, and the wire is ball bonded to the contact on the substrate (not shown in FIGS. 1 and 2). The wire is then fed through the capillary as it moves back over the ball bump 3 and the capillary stitch bonds the wire 4 directly to the top of ball bump 3.
Wire bonding equipment is often a bottleneck in the production of a microelectronic device assembly because a length of bonding wire must be separately attached to two different locations to create a single electrical connection between the die 1 and the substrate. Dies often have many bond pads which require wire bonding, with some dies requiring hundreds of separate wire bonds. Whereas conventional wire bonding requires two separate bonds for every wire, SSB requires that the capillary first formed a ball bump 3 before beginning the wire bonding process. This in essence requires the formation of three distinct bonds for each bonding wire. This can significantly add to the processing time required in a wire bonding operation.
SSB also requires very high precision in positioning the bonding wire 4 to form the stitch bond 5. The width WB of the ball bump 3 is generally correlated to the diameter of the bonding wire 4. As suggested in FIGS. 1 and 2, the ball bump 3 can be significantly smaller than the total available surface area of the bond pad 2. This reduces the room for potential error in positioning the wire with respect to the bond pad 2 to form the stitch bond 5.